Many of the following definitions pertaining to genetics are taken from a glossary developed and maintained by the National Human Genome Research Institute (NHGRI). The NHGRI glossary may be accessed at http://www.nhgri.nih.gov/DIR/VIP/Glossary/pub_glossary.cgi

ADLs

Activities of daily living; measures of basic functional status (e.g., bathing, dressing, toileting, continence, feeding, transferring). Sometimes referred to as BADLs (basic activities of daily living).

Allele

One of the variant forms of a gene at a particular locus, or location, on a chromosome. Different alleles produce variation in inherited characteristics, such as hair color or blood type.

Allostatic load

A composite measure of the body's accommodation to stress from the wear and tear resulting from chronic overactivity or underactivity of physiological systems.

Amino acid

Any of a class of 20 molecules that combine to form proteins in living things. The sequence of amino acids in a protein and hence protein function are determined by the genetic code.

Antagonistic pleiotropy

Multiple gene effects such that alleles that improve fitness in early life have detrimental effects later in life.

APOE

The apolipoprotein E gene, which instructs the body to make a protein that ferries cholesterol through the bloodstream. One variant (allele) of APOE (APOE e4) is associated with increased risk of certain diseases, notably heart disease and Alzheimer's.

Apoptosis

Programmed death of cells during embryogenesis and metamorphosis or during cell turnover in adult tissues.

Assortive mating

A process by which individuals select mates on the basis of one or more phenotypic characteristics.

Bacterial artificial chromosomes (BACs)

Large segments of DNA (100,000 to 200,000 bases) from another species cloned into bacteria. Once the foreign DNA has been cloned into the host bacteria, many copies of the DNA can be made.

Base pair

Two complementary nucleotides in double-stranded DNA.

Base sequence

The order of nucleotide bases in a DNA molecule.

Biodemography

An emerging field of research that seeks to integrate and translate findings from a variety of disciplines (e.g., demography, evolutionary and molecular biology, genetics, epidemiology, ecology) into their effects on population health status and individual/social behavior.

Bioindicators

In biology and ecology, living organisms that respond in a clear way to a change in the environment. Increasingly used in health and social sciences to refer to measurable properties of organisms that are associated with the manifestation of, or susceptibility to, disease processes.

Biomarkers

Observable properties of an organism that can be used in four general ways: (1) to identify the organism's presence, (2) to estimate prior exposure to an exogenous agent, (3) to identify changes in the organism, and (4) to assess underlying susceptibility.

Biomarkers of aging

Age-related biological parameters, the absolute values or rates of change of which might estimate, for example, subsequent life expectancy.

BRCA1/BRCA2

The first breast cancer genes to be identified. Mutated forms of these genes are believed to be responsible for about half the cases of inherited breast cancer, especially those that occur in younger women.

Candidate gene

A gene, located in a chromosome region suspected of being involved in a disease, whose protein product suggests that it could be the disease gene in question.

cDNA

A DNA sequence copied in the laboratory from a messenger RNA sequence.

Contamination effect

In the context of ongoing surveys, the possibility that a respondent's behavior will be affected by information revealed by the survey (e.g., if a study team discovers that an individual has a disease and relays that information to the individual, his/her subsequent behavior likely will be less representative of the larger population and more reflective of the intervention of the study itself).

Crossing over

The exchange of parts between homologous chromosomes during meiosis; recombination.

DNA

Deoxyribonucleic acid: the molecular basis of heredity.

DNA sequencing

Determining the exact order of the base pairs in a segment of DNA.

Enzyme

A protein that catalyzes or accelerates specific biochemical reactions.

EPESE

Established Populations for Epidemiologic Studies of the Elderly. Ongoing studies, initiated by the National Institute on Aging in 1982, of randomly sampled community-dwelling men and women aged 65 years or older. Major findings have focused on biomedical and psychosocial predictors of health and functioning in older cohorts.

Epigenetic changes

A broadly used term that historically described changes in gene expression or phenotype that were not the result of a mutation or other change in the DNA sequence of a gene. The term now includes specific changes (e.g., methylation of DNA bases) that can modify gene expression.

Epistatic effect

The suppression of gene expression by one or more other genes.

Exon

The region of a gene that contains the code for producing the gene's protein. Each exon codes for a specific portion of the complete protein. In some species (including humans), a gene's exons are separated by long regions of DNA called introns (or sometimes “junk DNA”) that have no apparent function.

F₁ generation

Offspring, from the mating of individuals of two different strains, that are heterozygous for all loci for which their parental strains differed in allelic configuration.

Founder effect

The introduction of a certain gene variant into a population such that the variation spreads in the absence of competition, which leads to fixation of the characteristics of that variant.

Gamete

An egg or sperm cell; a germ cell.

Gene

The functional and physical unit of heredity passed from parent to offspring. Genes are pieces of DNA, and most genes contain the information for making a specific protein.

Gene mapping

Determination of the relative positions of genes on a DNA molecule and of the distance, in linkage units or physical units, between them.

Genetic drift

Genetic changes in populations caused by random phenomena rather than by selection.

Genetic marker

A segment of DNA with an identifiable physical location on a chromosome whose inheritance can be followed. A marker can be a gene, or it can be some section of DNA with no known function.

Genome

The total inventory of heritable nucleic acids, usually DNA, including chromosomal DNA in cell nuclei (nuclear genome) but also mitochondrial and chloroplast DNA.

Genotype

The genetic inventory of an individual organism that may not be revealed by outward (observable) characteristics.

Gerontogene

A gene that affects longevity either by reducing (e.g., as a result of antagonistic pleiotropy) or increasing average life span.

Germ cell

A reproductive (i.e., egg or sperm) cell.

Health and Retirement Survey (HRS)

A longitudinal study begun in 1992 of 12,600 persons aged 51-61, plus their spouses, to which additional cohorts and age groups have been added in subsequent years. See http://www.umich.edu/∼hrswww/

Heritability

The proportion of the variation in the distribution of a quantitative trait that is explained by inherited genes.

Homeostasis

The maintenance of normal internal stability in an organism by coordinated responses of organ systems that automatically compensate for environmental changes.

Homocysteine

An amino acid, formed by the liver after ingestion of another amino acid (methionine), that participates in metabolic pathways for certain vitamins.

Homologous

Corresponding in structure and position; allelic chromosomes are homologous. Also refers to structures or processes that have the same evolutionary origin though their functions may vary widely.

Human Genome Project (HGP)

An international research project to map each human gene and to completely sequence human DNA.

IADLs

Instrumental activities of daily living; measures of an individual's ability to maintain an independent household (e.g., shopping for groceries, driving or using public transportation, using the telephone, meal preparation, housework, handyman work, laundry, taking medications, handling finances). Many community-based services (e.g., homemaker, meals-on-wheels) are aimed at providing IADL services and thereby permitting disabled older persons to remain in their homes.

Isoforms

For genes, isoforms can refer to alternatively spliced forms of the same gene. For proteins, isoforms can refer to members of a family of closely related proteins coded by different genes that evolved from a shared single ancestral gene.

Linkage

The association of genes and/or markers that lie near each other on a chromosome. Linked genes and markers tend to be inherited together.

Locus (pl. loci)

The set of homologous parts of a pair of chromosomes that may be occupied by allelic genes; the locus thus consists of a pair of locations (except in the X chromosome of males).

LOD score

“Likelihood of odds” score; a statistical estimate of whether two loci are likely to lie near each other on a chromosome and are therefore likely to be inherited together.

Longevity genes

Genes that promote survival. Most fixed genes are presumed to be of this type.

Meiosis

Two consecutive special cell divisions in developing germ cells, characterized by the pairing and segregation of homologous chromosomes. The resulting germ cells (gametes) will have reduced chromosome sets.

Messenger RNA (mRNA)

A variety of RNA that serves as a template for protein structure and synthesis. The sequence of a strand of mRNA is based on the sequence of a complementary strand of DNA.

Microarray

A recently developed technology based on computer chips for examination of hundreds to thousands of genes at the same time. The data may inform how the cell regulates batteries of genes simultaneously. Sometimes referred to as a gene chip.

Mitochondria

The subcellular particles (organelles) that make the ATP (energy) used for most cell activities. Mitochondria have a small number of their own genes.

Monogenic diseases

Diseases caused by mutation in a single gene.

Monozygotic twins

Identical twins, who share all their genetic material because they came from a single egg that separated into two embryos after fertilization by a single sperm.

Morphogens

Molecules that function as pattern organizers in evolving tissues (e.g., in embryos), signaling and directing gene activity in their target cells.

NHANES

The National Health and Nutrition Examination Survey; an ongoing program of studies conducted by the National Center for Health Statistics and designed to assess the health and nutritional status of adults and children in the United States. See http://www.cdc.gov/nchs/nhanes.htm

Nucleotide

One of the structural components of DNA and RNA. A nucleotide consists of a base (one of four chemicals: adenine, thymine, guanine, and cytosine) plus a molecule of sugar and one of phosphoric acid.

Oligogene

Also called a major gene, thought to have moderate-to-large effects on a given phenotypic trait.

Oligonucleotide

A short sequence of single-stranded DNA or RNA often used as a probe for detecting complementary DNA or RNA because it binds readily to its complement.

Pedigree

A recorded or known line of descent. In humans, this may be a diagram of family genealogy that shows family members' relationships to each other and how a particular trait or disease has been inherited.

Phenotype

The physical manifestation of gene function; the observable traits or characteristics of an organism (e.g., hair color, weight, the presence or absence of a disease). Phenotypic traits can be influenced by the environment.

Phyletic

Of, or pertaining to, a phylum or to an evolutionary line of descent.

Pleiotropy

Multiple effects of one gene; the capacity of a gene to affect several aspects of the phenotype.

Polygenic

Relating to a normal characteristic or hereditary disease controlled by the added effects of genes at multiple loci.

Polymerase chain reaction (PCR)

A widely used and convenient technique for using enzymes to make an unlimited number of copies of any piece of DNA. Sometimes called “molecular photocopying.”

Polymorphism

A gene variation (allele) present in at least 1 percent of the population.

Positional cloning

Identification of a gene by virtue of its location in the genome rather than by its biochemical function.

Probands

People who have the disorder under investigation in a family history study.

Proteomics

The study of the entire protein set that can be produced by a given genome; also understood more broadly as the large-scale experimental analysis of proteins.

Quantitative trait

A trait for which phenotypic variation is continuous (rather than discrete).

Quantitative trait locus (QTL)

One of a group of genes specifying any particular quantitative trait.

Recombinant

A composite DNA molecule resulting from insertion into the original sequence (by chemical, enzymatic, or biologic means) of a new sequence; an offspring that has received chromosomal parts from different parental strains.

Recombination

Reshuffling of parental genes during meiosis due to crossing over. Recombination also may be induced in a test tube by enzymes.

RNA

Ribonucleic acid, a single-stranded string of cytocine, guanine, adenine, and uracil bases.

Senescence

Deterioration in performance seen later in the adult life span, associated with increasing mortality rates.

Short tandem repeat (STR)

A tandem repeat in which the repeat units are 2-7 base pairs. Also referred to as a microsatellite.

Single nucleotide polymorphism (SNP)

A single-base variation in the genetic code, the most common form of polymorphism.

Somatic cells

All cells in the body except for the germ-line cells that make eggs or sperm.

Stochastic

Random; involving chance or probability.

Syntenic

Denoting the linkage of a group of genes that is found in related species.

Tandem repeat

Multiple copies of the same base sequence on a chromosome; used as a marker in physical mapping.

T-cell

Also called t-lymphocyte, a white blood cell that is essential in the immune system.

Telomere

A specialized repeated DNA sequence found at the end of a chromosome.

Variable number of tandem repeats (VNTRs)

Repeating units of a DNA sequence.

Zygote

The cell resulting from the union of a male and female gamete until it divides; the fertilized ovum.

Zygotic twins

Nonidentical (fraternal) twins, who on average share 50 percent of their genes (similar to ordinary siblings).